Vacuum-assisted resin transfer molding (VARTM) is being used more frequently to mold large composite structures, such as sections of aircraft. VARTM is a variant of traditional resin transfer molding (RTM), possessing advantages over conventional RTM by eliminating matched-metal tooling costs, reducing volatile emissions and allowing for low injection pressures. In VARTM, vacuum pressure is used to force liquid resin into dry composite reinforcements that have been laid in a sealed mold, often in the form of a preform. The mold can be a one sided tool with a vacuum bag, a two sided matched tool with a vacuum seal, or a soft bag enclosing the entire structure to be molded. Vacuum pressure is used to pull or drive resin into the mold, thus VARTM is sometimes referred to as a vacuum infusion process. The selection of materials, arrangement of mold gates/vents and the selection of processing parameters often have a significant impact on product quality and process efficiency in VARTM.
When molding relatively large structures, such as an aircraft fuselage, gravitational effects on resin flow behavior can create undesirable thickness gradients in the finished structure. These gradients, which may approach 25% or more, result from the fact that the force imposed by gravity tends to draw the flowing resin downwardly toward the bottom of the molded structure during the curing process, until the resin is sufficiently cured to terminate its flow. As a result, wall thickness of the structure measured in bottom portions of the structure can be significantly greater than wall thickness near the top of the structure. Thickness gradients due to resin migration not only reduce the integrity of the molded structure, but also result in a structure that is unnecessarily heavy, since in order to achieve a minimum wall thickness at the top of the structure, wall thickness near the bottom of the structure is greater than necessary. In the case of aircraft structures, thickness gradients of the type described above make it difficult to produce complete fuselage sections having integral stringers and co-bonded fuselage frames.
Accordingly, there is a need in the art for an improved method and apparatus for manufacturing composite structures using VARTM which overcomes the deficiencies of the prior art discussed above. The present invention is directed to satisfying this need.